1. Being captivated by someone's buttocks or having one's attention grabbed by looking at someone's bottom. One can be fassinated or find someone's ass fassinating or just caught in fassination.
2. Fantasizing about someone's ass.

When Zeta saw Zatty's bum, he became totally fassinated. The fassination with her bottom persisted all daylong. He would often tell her that he found her fassinating, but she didn't really know what he had in mind.